Ion mirrors, or reflectrons, are components used in mass spectrometer systems to reverse or redirect the trajectory of ions as they travel toward a detector within a mass analyzer. In particular, ion mirrors are often used in Time-of-Flight (TOF) mass spectrometers where they are placed at the end of a drift region. FIG. 1 depicts a conventional ion mirror 5 with top portions cut away for illustrative purposes. A series of electrically conductive electrode plate elements 10, which can vary in number, are arranged spaced apart in the axial direction by insulating spacer elements 15. As shown, the electrode plate elements 10 are configured as rectangular rings enclosing a central ion conduit region 20 through which ions travel axially. The electrode plate elements 10 can also be configured as circular annular rings. In addition to electrode plate elements 10, one or more grid elements 25 are arranged perpendicular to the axis of the ion mirror 5. Voltages applied to the electrode elements and grid elements generate a retarding electric field within the ion conduit region 20. In gridded mirrors, electrode plate elements 10 are typically spaced evenly and the applied voltages are derived from resistor stacks of equal value, generating a constant field. The grid elements 25, coupled to separate voltage sources, function as borders between regions having different electric fields. Grid elements 25 placed at the ends of the ion conduit region 20 terminate the fields so that the fields within the ion conduit region 20 exert no forces outside of the ion mirror 5. Gridless ion mirrors are also used, in which electrode plate elements may or may not be equally spaced, but are usually tuned with various voltages not simply derived from linear resistor networks.
In either case, in typical orthogonally pulsed instruments, the ions often enter the mirror with a natural angle with respect to the longitudinal axis of the mirror based on the ratio of the pulsing energy to the ion source energy, and the mirror is placed parallel to the pulser. As shown in FIG. 1, the ions then exit the mirror with approximately the same angle to the longitudinal axis as if reflected from the entrance of the mirror.
Ion mirrors can be used advantageously to improve the mass resolution of TOF mass spectrometers. Typically, the mass resolution of TOFs is limited by such factors as uncertainties in: the time when the ions were pulsed (time distribution); their location in the accelerating field when pulsed (spatial distribution); and variation in initial kinetic energies prior to acceleration (energy distribution). The spatial distribution of ions in the pulsing region is associated with an energy distribution that leads directly to a corresponding time distribution in the time the ions reach the detector. If properly designed, a reflectron ion mirror can compress the time distribution caused by the initial pulser space distribution. This is possible because with larger kinetic energies, ions penetrate the retarding field more deeply before being turned around. These “faster” ions catch up with the slower ions at the detector. Effectively, the initial spatial distribution can be reduced by an order of magnitude at the crucial time when the ions hit the detector. Thus the initial spatial distribution need not compromise a desired high temporal resolution.
One of the prerequisites for a high degree of improvement in temporal resolution is that the equipotential lines of the retarding electric field within the ion conduit region must be parallel across the width of the ion packet as it travels the through the ion mirror. Although instruments typically have only a few ions in every pulse, it is nevertheless useful to conceptualize an ion packet that is the summation of many consecutive pulses. FIG. 2 schematically illustrates an axial section of an ion mirror in which the equipotential lines 40a are parallel. It is found that generating and maintaining parallel equipotential lines places high mechanical tolerances on both the electrode elements and the insulating spacers. In particular, systematic errors in the sizes of the electrode plate elements can cause an ion mirror assembly to expand or contract along its axis. If “n” plate elements are used, then non-random errors in plate size must be 1/nth of the amount of drift that can be tolerated in the assembly as a whole. Furthermore, cumulative errors can build up if the insulating spacers are not precisely dimensioned. FIG. 3 schematically illustrates the effect that such systematic errors and other commonly occurring inaccuracies, such as misalignment, can have on the contour of equipotential lines within the ion conduit region of an ion mirror. As shown, equipotential lines 40b are not parallel. Ion packets traveling axially will be subject to different electric fields depending upon their radial location within the conduit bore. Accordingly, the spatial distribution and time distribution of the ion packet will tend to broaden, canceling the spatial and temporal focusing effects of the electric fields applied in the ion mirror. To avoid the deleterious consequences of inaccuracies in plate and spacer dimensions, pre-measuring and sorting can be performed to compensate for the systematic errors and drifts in plate and spacer size. However, these operations involve significant part and labor costs.